The basal ganglia are a group of subcortical nuclei that are important for motivation and motor control. Disorders of the basal ganglia lead to a variety of disabling movement disorders, the most common of which is Parkinson's disease. The input nuclei of the basal ganglia in primates include the caudate and putamen. The output nuclei include the Gpi and the substantia nigra pars reticulata. Other important nuclei include the substantia nigra pars compacta and the subthalamic nucleus. The input and output nuclei of the basal ganglia are joined by two distinct sets of connections, known as the "direct" and "indirect" pathways. The current model of basal ganglia function holds that the two pathways are in functional opposition and that activation of the direct pathway facilitates movement while activation of the indirect pathway inhibits movement. This explanation works well in empirically explaining what areas of the two pathways are overactive in movement disorders. For example, the Gpi and STN are overactive in PD and hence are effective targets for treatment. However, the nature of the interaction between the two pathways is poorly understood. Most recent models of the basal ganglia emphasize their role in suppressing unwanted movements although this has never been directly tested. Therefore, the primary goal of this research is to understand the role of the basal ganglia in suppressing unwanted movements by recording the activity of basal ganglia neurons in awake behaving primates trained in a movement suppression task. The second goal is to compare the data obtained in primate studies with information obtained by recording from the subthalamic nucleus and globus pallidus of patients undergoing surgery for the treatment of Parkinson disease. In this fashion we hope to understand the derangements of basal ganglia function which occur in PD and to devise better treatment strategies. This work will be conducted in the Department of Neurobiology at Harvard Medical School and in the Department of Neurosurgery at Massachusetts General Hospital.